This invention relates to field sequential light valve projection display systems, and more particularly relates to such systems in which the display panel is addressed one line at a time.
Light valve projectors generally achieve a full color display by first breaking white light into red, green and blue components. These color components are then modulated by one or more light valves in accordance with the red, green and blue components of the color display signal. The display signal components are divided into fields of information, in the manner of video broadcast signals, and, in the case of a single light valve projector, the red, green and blue fields are sequentially applied to the light valve. The RGB cycle repeats at a frequency to give a viewer the impression of a steady, flicker-free image, for instance, 60 Hz for USA television.
A typical light valve is a two-dimensional matrix of individually-addressable pixel elements, and the most common method of loading a field of information onto the light valve, the so-called "line-at-a-time" method, is to address the matrix one row or line of pixels at a time. (In the less common "field-at-a-time" method, the data for an entire field is loaded into a "shadow RAM" and then transferred to the entire matrix of pixels at one time). In the line-at-a-time scheme, as soon as data is loaded into a row or line of pixels, the data affects the display. The drive electronics then select the next line for addressing, which is generally immediately adjacent to the previously-addressed line.
If each pixel of the light valve is driven to its maximum transmission (or reflection) state for each of the three colors, a solid white field of the maximum brightness the projector is capable of achieving will be displayed at the viewing screen. For any given application, there is a target white value, which can be represented as a point on the CIE chromaticity diagram. For instance, for consumer television in the USA and Europe, the target point is D65 white (corresponding to CIE color coordinates of x=0.313, y=0.329). If the white achieved by the maximum drive for all three colors matches this target value, no color correction is necessary. However, in practice, the light source will generally be found to be deficient in one or more colors, and some color correction will be required.
The simplest way to achieve color correction is to discard the light that is in excess. For one type of high intensity discharge lamp which is red-deficient, it would then be necessary to discard some blue and green to achieve the correct color balance. Discarding blue has little effect on the lumen throughput, but discarding green can have a dramatic effect. Sometimes, depending on projector design, it may be necessary to discard almost half of the green light. This can reduce lumen throughput sufficiently to render the projector non-competitive.
In one type of field-sequential, line-at-a-time color projection system, described in commonly assigned U.S. Pat. No. 5,532,763, white light from a source is divided into three color components or subbeams having band-shaped cross-sections. These subbeams are scrolled sequentially across the light valve, eg, an LCD panel, while those portions of the panel which are illuminated by the bands are synchronously addressed with the corresponding display signal information. Such projection systems are referred to herein as single panel scrolling raster (SPSR) projectors.
A virtue of the SPSR projector is that it is possible to make the height and spacing of the subbands sufficiently small that two or even three of the subbands are present on the panel at the same time. Thus, the efficiency of such a projector can be two or three times that of prior field sequential projectors in which only one color, or one third of the total light, can be present on the panel at one time.
One way of achieving color correction in such a SPSR system, described in commonly assigned U.S. Pat. No. 5,548,347, is to increase the light throughput of the color that is deficient while simultaneously decreasing the throughput of the color that is excessive, by changing the band dimension in the scrolling direction, ie, increasing the band for the deficient color and decreasing the band for the excessive color.
A different type of SPSR color projection system is described and claimed in commonly assigned U.S. patent application Ser. No. 09/127003, filed Jul. 31, 1998, the entire specification of which is incorporated herein by reference. In this projector, white light from a source is incident on a single rotationally-symmetric element such as a drum, having reflective surface portions with different color reflection bands. The surface portions separate the white light into the desired color bands and reflect the bands into a relay lens, which images the bands onto the light modulator panel, while the rotation of the drum causes the color bands to scroll across the panel in the desired manner.
Such a projection system eliminates the need to form the white light into subbands prior to scrolling, and thus makes possible a smaller and less expensive light valve panel, and correspondingly smaller and less expensive optical components.